Coupling circuits



Oct. 31, 193%. F. A. LElBE 2,178,453

COUPLING CIRCUITS Filed Dec. 31, 1957 IL j] UNBALANCED BALANCED um: 25 L225 Ns1 vsoak NETWORK AVAVA lNl/E/VTOR FA.LE/BE ATTORNEY Patented Oct. 31, 1939 UNITED STATES mrsnr orries Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application December 31, 1937, Serial No.'-182,658

17 Claims.

This invention relates to coupling circuits and more specifically to circuits for coupling an unbalanced line or network and a balanced line or network.

In the interconnection of a coaxial cable, one element of which is customarily connected to ground or to a point of fixed potential, and a cable pair, the elements of which are customarily balanced with respect to ground or to a point of fixed potential, coupling circuits are necessary. For the transmission of television programs, the television terminal equipment now being contemplated by a number of investigators makes use of a very wide range of frequencies extending from a few cycles to a million or more cycles. This range is beyond that which can be efficiently handledby transformers which can be built at the present time. The solution of the problem of interconnecting balanced and unbalanced circuits without the use of transformers or coils is therefore an important one.

Coupling circuits are known which interconnect a balanced and an unbalanced circuit without the use of transformer coils. In each case, however, the circuit is suitable for transmission in one direction only, that is, from the balanced circuit to the unbalanced circuit or from the unbalanced circuit to the balanced circuit, but not from one to the other as desired without rearrangement of the coupling circuit.

It is an object of this invention to provide a circuit for the interconnection of a balanced and an unbalanced circuit without the use of coils, which may be used for transmission in either direction.

In accordance with the invention, a circuit is provided for interconnecting an unbalanced circuit and a balanced circuit comprising an electron discharge device used as a phase inverter, and an impedance network. The elements comprising the network are preferably so chosen that the impedances of the circuit at high frequencies are matched in both directions. This coupling circuit is capable of transmitting an extremely wide range of frequencies, such as, for example, that used in television transmission. In general, the high frequency impedance of a transmission line is substantially a pure resistance. If a transmission line is terminated in this value of resistance, reflections are caused at low frequencies, but these reflections are ordinarily of such a nature as to render the transmission efficiency of the line more nearly uniform over the whole frequency range. The delay distortion of the 5;; over-all circuit is also reduced under these conditions. It follows that the simplification of the coupling circuit provided by the use of resistive elements will not usually entail any disadvantages in its operation.

The invention will be more readily understood from the following "description taken in connection with the accompanying drawing forming a part thereof in which the single figure shows a circuit for coupling an unbalanced to a balanced circuit and vice versa, without the use of transformer coils.

Referring more particularly to the drawing, the single figure shows a box II] representing generally an unbalanced line or network, a box II representing generally a balanced line or network, and a circuit I2 for interconnecting the unbalanced line It and the balanced line II.

The interconnecting circuit preferably comprises an electron discharge device I3, comprising an anode I4, a cathode I5, a control grid IS, a screen grid II, and a suppressor grid I8, which device is used as a phase inverter, and an impedance network comprising the resistance elements I9, 20, 2| and 22.

The control grid I6 of the electron discharge device I3 is connected through an appropriate coupling condenser 23 to the ungrounded terminal 24 of the unbalanced line or network II]. The cathode I5 has one terminal connected to ground, the suppressor grid I8 being also grounded. Between the control grid I6 and one terminal of the cathode I5 are connected a conventional biasing resistance 25 and a source of potential 26. Connected between the anode I l and the cathode I5 are an anode resistance 21 and a source of anode potential 28." The source 28 or a portion thereof also serves as a means to positively bias the screen grid IT with respect'to the'cathode I5. A source 29 supplies heating current to the cathode I5. The anode I4 of the'electron discharge device I3 is coupled to one terminal 30 of the balanced line or network II bymeans of the coupling condenser 3|. One terminal of the cathode I5 is connected to the terminal 30 (through condenser 3 I) by means of the resistance element I9, and this terminal of the cathode is also connected to the other terminal 32 of the network II by means of the resistance elements 20 and 22 in series. The terminalz l of the unbalanced line or network II! i's'connected to the terminal 32 of the balanced line or network II by means of resistance elements 2! and 22 in series.

Considering the transmission in the direction from the unbalanced line or network In to the balanced line or network II, the phase of the line current at the terminal 32 is the same as that of the line current at the terminal '24 because no phase change occurs in the passage of current through the resistance elements 2| and 22 in series. The phase of the line current at the terminal 30 is substantially 180 degrees from that of the line current at the terminal 2t and hence of the line current at the terminal 32 because of the phase inverting action of the electron discharge device I3. The terminals 30 and 32 are balanced with respect to ground by means of the resistances 20 and 22 connected in series between the terminal 32 and ground, the resistance 20 being shunted by the series circuit comprising resistance 2|v and the impedance of the unbalanced line or network I0, and the resistance I9 in parallel with the series circuit comprising the condenser 3I and the anodecathode impedance path of the discharge device I3 shunted by the series circuit comprising the resistance 21 and the source 28. The values of the resistances 19, 2|], 2|, 22 and 21 are chosen with respect to that of the constants of the lines and of the electron discharge device I3 and its associated input and output circuits so that the impedances are matched in both directions of transmission between the unbalanced and balanced circuits.

Considering the transmission from the balanced line or network II to the unbalanced line or network 10, the line current at the terminal 30 assumes the phase of the line current at the terminal 33 inasmuch as the impedance of the circuit represented by the resistance I9 and its parallel paths is resistive. No current, of course, flows from the anode I4 to the cathode I5 when transmission is from the balanced line I I to the unbalanced line I 0. The line current at the terminal 32 is in phase with that at the terminal 24 due to the pure resistance load H and 22.

The values of the elements of the circuit may be determined in the following manner. The reactance of the coupling condenser 23 at the lowest frequency to be transmitted should be small compared with the resistance of element 25. The resistance of the resistor 25 is not critical, but may conveniently be made very large compared with the impedance of the unbalanced line or network III. The reactance of the coupling condenser 23 at the lowest frequency to be transmitted is preferably small compared with the resistance I9. The impedance of the resistance element of element 21 is not critical, but may be made large compared with resistance element I9; this condition yields a smaller loss through the circuit and simplifies the determination of the values of the remaining elements. The electron discharge device I3 may be a conventional pentode vacuum tube, in which case its anode-to-cathode impedance would be large compared with that of the resistance I9. With these approximations, the impedance across the terminals of the resistance IS with the balanced line or network I I removed is substantially equal to the resistance of I9.

Let the high frequency impedance of the unbalanced line be represented by Ru, of the balanced line by RB, and the resistances of elements I9, 20, 2| and 22, by R19, R20, R21, and R22, respectively. The impedance across terminals 30 and 32 when the balanced line is removed should equal RB, and since this impedance must be balanced to ground, R19 must equal By the same reasoning, the impedance from the right side of the T network, composed of elements 20, 2| and 22, and terminated with RU, must equal Similarly, the impedance across terminals 24 and 33 when the unbalanced line is. removed should equal Ru, which is the impedance from the left side of the figure of the T network terminated with That this latter value of termination is correct may be seen by the following reasoning: the balanced line or network II may be replaced by two resistances each equal to and connected in series; the junction between them will be at ground potential, and may therefore be connected to ground by a wire without disturbing the existing distribution of currents and voltages.

Consider the direction of transmission to be from the unbalanced to the balanced line, and let the voltage across terminals 24 and 33 be E. Then the vacuum tube may be replaced by a constant current generator of value EG where G is the mutual conductance of the vacuum tube, as described in Chapter IX of the book The Theory of Electrical Artificial Lines and Filters by A. C. Bartlett. This constant current generator is connected to the equal resistances R19 and in parallel, and hence the voltage across 2 equals 1 EGR The voltage E also appears across the left side of the T network and the values of R20, R21, and R22 must be chosen so that the voltage 1 EGR appears across its termination which is of value It may happen that for certain combinations of values of Ru, RB, and G, a non-physical solution will be obtained for certain of the elements R20, R21, and R22. In this event a different vacuum tube may be chosen, or the effective mutual conductance of the tube may be reduced in any wellknown manner, so that the solutions of the equations yield attainable values for the circuit elements.

When the above conditions are met, the loss occurring in the complete circuit when transmitting from the unbalanced to the balanced circuit is equal to (SL6866-3) decibels. For transmission'from the balanced to the unbalanced circuit, the loss is. (8.6869+$) decibels.

As a specific example consider the interconnection of a Vii-ohm unbalanced circuit with a GOO-ohm balanced circuit for a frequency range lfrom a few cycles to about two million cycles,-

matching the impedances in both directions. The pentode vacuum tube used has a mutual conductance of 3000 micro-amperes per volt. The

resistance !9 has a value of about 300 ohms, re- .sistance 2!) a value of 71.1 ohms, resistance 2! of circuit having terminating impedances which-are resistive and are constant over a wide frequency range. This is not a necessary. condition, for a suitable choice of impedance values 'forelements 28, 2B and 22,. and the substitution of a similar arrangementfor the single element 19, may be employed to yield terminating impedances which vary with frequency in almost any desired Other modifications may obviously be made to suit various working conditions without departing from thespirit of the invention, the scope of which is defined by the a pended claims.

What is claimed is:

1. In a high frequency transmission system, a

circuit substantially balanced with respect to a fixed potential, a circuit unbalanced with respect to said fixed potentialpan d noneinductive means coupling said circuits in energy transfer'relation,

said coupling means comprising an'impedance network connected tothe balanced circuit for transferring energy from the balanced circuit to energy in cooperation with said network "from the unbalancedcircuit to the balanced circuit. 2. In a high frequency transmission system, a

circuit substantially balanced withrespectto a fixed potential, a circuit unbalanced with respect 'to saidlfixed potential, and means couplingsaid circuits in energy transfer relation, said coupling means comprising an arrangement including two impedance elements connectedin series, and thus "having a common terminal. for connecting the terminal of said unbalanced circuit which is not at a fixed potential to, one of theterminals of said balanced circuit not. at said fixed potential, a phase inverting deviceforconnecting the terminal of said unbalanced circuit which is not connected to a fixed potential to the other terminal of the balanced circuit not at said fixed potential, and a third impedance element connected between said other terminal of said balanced circuit and the common terminal. of said impedances in series.

3. In a high frequency transmission system, a

circuit substantially balanced with respect to a fixed potential, a circuit unbalanced with respect to said fixed potential, and means coupling said circuits in energy transfer relation, said coupling means comprisingan arrangement including two resistance elem'entsconnected in series,and thus having a common terminal, for connecting the terminal of said unbalanced circuit which is "not at a fixed potential to one of the terminals of said balanced circuit not at said fixed potential,

a phase inverting device for connecting the terminal of said'unbalanced circuit which is not connected to a fixed potential to the other terminal of the balanced circuit not at said fixed potential, and a third resistance element connected between said other terminal of said balanced circuit and the common terminal of said resistance in series.

4. In a high frequency transmission system, a circuit substantially balanced with respect to a fixed potential, a circuit unbalanced with respect to said fixed potential, and non-inductive means coupling said circuits in energy transfer relation, said coupling means comprising an electron discharge device having an anodeQa. cathode and a grid, means for connecting the terminal of said unbalanced circuit which is not at a fixed potential to said grid, means for connecting said terminal of said unbalanced circuit through an impedance to one terminal of said balanced circuit not at said fixed potential, means for connecting said anode to the other terminal of said balanced circuit not at said fixed potential, and means for connecting said cathode to said point of fixed potential.

5. In a high frequency transmission system, a circuit substantially balanced with-respect to a fixed potential, a circuit unbalanced with respect to said fixed potential, and means coupling said circuits in energy transfer relation, said coupling means comprising an'electrondischarge phase inverting means and an impedance network connected between said balanced circuit, said unbalanced circuitand said phase inverting means in such a manner that energy can be transferred in both directions between said circuits.

6. In a "high frequency transmissionsystem, a

circuit substantially balanced with respect to a fixed potential, a circuit unbalanced with respect to said fixed potential, and means'c'oupling said circuits in energy transfer relation, said coupling means comprising a phase inverting means and an impedance network connected between said balanced circuit, said unbalanced" circuit and said phase inverting means in such a manner that energy can be transferred in both directions between said circuits, said impedance network comprising only elements which have almost entirely resistive impedance.

'7. In a high frequency transmission system, a circuit substantially balanced with respect to a fixed potential, a circuitunbalancedwith respect to said fixed potential, and non-inductivame'ans coupling said circuits in energy transfer relation, said coupling means comprising an impedance network connected to the balanced circuit for transferring energy from the balanced circuit to the unbalanced circuit andan electron discharge phase inverting means ;connected to at least a portion of said network and to said unbalanced circuit for transferring energy in cooperation with said network from the" unbalanced circuit to the balancedicircuit. V

8. In a high frequency transmission system, a circuit substantially balanced with respect to a fixed potential, a circuit unbalanced with respect to said fixed potential, and means coupling said circuits in energy transfer relation, said coupling means comprising an electron discharge phase inverting means having an anode, a cathode, a control grid, a screen grid and a suppressor grid,

means for connecting said cathode and said screen grid to a point at said fixed potential, means for positively biasing said screen grid with respect to said cathode, means for connecting said anode to said cathode through an impedance and a source of fixed potential, means for connecting said anode to one of the terminals of said balanced circuit not at said fixed potential, an impedance network comprising an impedance element and a T-shaped impedance network member, means for connecting the impedance element of said impedance network between one terminal of said balanced circuit not at a fixed potential and said point of fixed potential, and means for connecting the terminals of said T- shaped network member respectively to said point of fixed potential, the other terminal of said balanced circuit not at said fixed potentiaL and the terminal of said unbalanced circuit not at said fixed potential.

9. A circuit for interconnecting a circuit substantially balanced with respect to a fixed potential and a circuit unbalanced with respect to said fixed potential, comprising an electron discharge device having input and output circuits, means for connecting the input circuit of said device to the terminals of said unbalanced circuit, means for connecting the output circuit of said device between one outside terminal of said balanced circuit and a point of said fixed potential, a T- shaped impedance network, and means for connecting the terminals of said network respectively to the terminal of said unbalanced circuit not at fixed potential, to the other outside terminal of said balanced circuit, and to said point of fixed potential.

10. A circuit for interconnecting a circuit substantially balanced with respect to a fixed potential and a circuit unbalanced with respect to said fixed potential, comprising an electron discharge device having input and output circuits, a T- shaped impedance network, means for connecting the input circuit of said electron discharge device to the terminals of said unbalanced circuit, an impedance element connected between one outside terminal of said balanced circuit and said point of fixed potential, means for connecting the output circuit of said electron discharge device to said impedance element, and means for connecting the terminals of said T-shaped network respectively to the other outside terminal of said balanced circuit, to the terminal of said unbalanced circuit not at said fixed potential, and to said fixed potential.

11. A circuit-for interconnecting a circuit substantially balanced with respect to a fixed potential and a circuit unbalanced with respect to said fixed potential, comprising an electron discharge device having input and output circuits, a T-shaped impedance network comprising elements which have almost entirely resistive impedance, means for connecting the input circuit of said electron discharge device to the terminals of said unbalanced circuit, an impedance element connected between one outside terminal of said balanced circuit and said point of fixed potential, means for connecting the, output circuit of said electron discharge device to said impedance element, and means for connecting the terminals of said T-shaped network respectively to the other outside terminal of said balanced circuit, to the terminal of said unbalanced circuit not at said fixed potential, and to said fixed potential.

12. A circuit for interconnecting a circuit substantially balanced with respect to a fixed potential and a circuit unbalanced with respect to said fixed potential, comprising an electron discharge device having input and output circuits, a T- shaped impedance network comprising elements which have almost entirely resistive impedance, means for connecting the input circuit of said electron discharge device to the terminals of said unbalanced circuit, an impedance element connected between one outside terminal of said balanced circuit and said point of fixed potenj tial, means for connecting the output circuit of said electron discharge device to said impedance element, and means for connecting the terminals of said T-shaped network respectively to the other outside terminal of said balanced circuit, j

to the terminal of said unbalanced circuit not at said fixed potential, and to said fixed potential, the said impedance and the elements comprising said T-shaped impedance network being so chosen with respect to the constants of said unbalanced and balanced circuits and of said electron discharge device and its associated input and output circuits that the impedances are matched in both directions of transmission between said balanced and unbalanced circuits at high frequencies.

13. A coupling circuit for coupling a balanced line having a first, a second, and a third terminal, the potentials of the first and second terminals of which are varying but are always balanced with respect to the potential of the third terminal which is maintained at a fixed value, with a two-terminal line, the potential of a first terminal of which is maintained at said fixed potential and the potential of a second terminal of which varies with respect to said fixed potential, said coupling circuit comprising a phase inverter having input and output circuits, the terminals of said input circuit being connected respectively to the terminals of said unbalanced line and the terminals of said output circuit being connected respectively to said first and third terminals of said balanced line, an electrical connection between said second terminal of said balanced line and said second terminal of said unbalanced line, a T-impedance network having its first and second arms in series in said last-mentioned connection and the outside terminal of its third arm connected to said third terminal of said balanced line, and an impedance connected between said first and third terminals of said balanced line.

14. A coupling circuit-for coupling a balanced line having a first, a second, and a third terminal, the potentials of the first and second terminals of which are varying but are always balanced with respect to the potential of the third terminal which is maintained at a fixed value, with a two-terminal line, the potential of the first terminal of which is maintained at said fixed potential. and the potential of the second terminal of which varies with respect to said fixed potential, said coupling circuit comprising a phase inverter having input and output circuits, the terminals of said input circuit being connected respectively to the terminals of said unbalanced line and the terminals of said output circuit being connected respectively to said first and third terminals of said balanced line, an electrical connection between said second terminal of said balanced line and said second terminal of said unbalanced line, a T-impedance network having its firstand second arms in series in said lastmentioned electrical connection and the outside terminal of its third arm connected to said third terminal of said balanced line, and an impedance connected between said first and third terminals of said balanced line, said phase inverter being an electronic vacuum tube repeater, the input terminals of which are coupled to said unbalanced circuit terminals through a series capacity and a shunt resistance and the output terminals of which are similarly coupled to said first terminals of said balanced line.

15. A coupling circuit for coupling a balanced line having a first, a second, and a third terminal, the potentials of the first and second terminals of which are varying but are always balanced with respect to the potential of the third terminal which is maintained at a fixed value, with a two-terminal line, the potential of the first terminal of which is maintained at said fixed potential, and the potential of the second terminal of which varies with respect to said fixed potential, said coupling circuit comprising a phase inverter having input and output circuits, the terminals of said input circuit being connected respectively to the terminals of said unbalanced line and the terminals of said output circuit being connected respectively to said first and third terminals of said balanced line, an electrical connection between said second terminal of said balanced line and said second terminal of said unbalanced line, a T-impedance network having its first and second arms in series in said last-mentioned electrical connection and the outside terminal of its third arm connected to said third terminal of said balanced line, and an impedance connected between said first and third terminals of said balanced line, all of the impedances in said coupling circuit being substantially non-reactive.

16. A coupling circuit for coupling a balanced line having a first, a second, and a third terminal, the potentials of the first and second terminals of which are varying but are always balanced with respect to the potential of the third terminal which is maintained at a fixed value, with a two-terminal line, the potential of the first terminal of which is maintained at said fixed potential and the potential of the second terminal of which varies with respect to said fixed potential, said coupling circuit comprising a phase inverter having input and output circuits, the terminals of said input circuit being connected respectively to the terminals of said unbalanced line and the terminals of said output circuit being connected respectively to said first and third terminals of said balanced line, an electrical connection between said second terminal of said balanced line and said second terminal of said unbalanced line, a T-impedance network having its first and second arms in series in said last-mentioned electrical connection and the outside terminal of its third arm connected to said third terminal of said balanced line, and an impedance connected between said first and third terminals of said' balanced line, said impedances being so chosen for a suitable range of frequencies that the impedance between the first and third terminals of said balanced line with the line removed looking towards said unbalanced line is equal to one-half of the approximate line impedance between the first and second terminals of said balanced line, the impedance looking towards the unbalanced line from between the second and third terminals of the balanced line with the latter removed is also equal to one-half of the approximate line impedance between the first and second terminals of said balanced line, and the impedance between said terminals of said unbalanced line with the latter removed looking toward the balanced line is equal to the approximate line impedance of said unbalanced line looking into said unbalanced line.

17. In a high frequency transmission system, a circuit substantially balanced with respect to a fixed potential, a circuit unbalanced with respect to said fixed potential, and means coupling said circuits in energy transfer relation, said coupling means comprising an arrangement including two impedance elements connected in series,

and thus having a common terminal, for connect ing the terminal of said unbalanced circuit which is not at a fixed potential to one of the terminals of said balanced circuit not at said fixed potential, a phase inverting device for connecting the terminal of said unbalanced circuitwhich is not connected to a fixed potential to theother terminal of the balanced circuit not at said fixed potential, and a third impedance element connected between said other terminal of said balanced circuit and the common terminal of said impedances in series, said third impedance element having an intermediate terminal which is placed at said fixed potential.

FRANK A. LEIBE. 

